XXXXXXX = random name
all these mail addresses didnt exist of course
but they can send spam (to aol, gmail, etc.)

I tried to block sending mail from domain, but it didnt work. (it works only when i send the mail from my existing mail address, but the spamers still can send spam...)
this link : serverfault . com / questions/517945/how-to-block-sending-mail-from-domain-in-postfix